Preselector system for radio receivers



March 28, 1939.

P. A. LE MARCHANT 2,151,780

I PRESELECTOR SYSTEM FOR RADIO.RECEIVERS Filed Jan. 21, 1937 lAlIE/WM. Pmzy A LE Mammal-r,

Patented Mar. 28, 1939 1 UNITED STATES PATENT OFFICE PRESELEOTOR SYSTEM FOR,

RECEIVERS RADIO a British company Application January 21, 1937, Serial No. 121,622 In Great Britain January 31, 1936 9 Claims.

The present invention relates towireless and like receivers, and is particularly concerned with receivers of the superheterodyne type.' In re- I ceivers of this type received carrier oscillations representing a desired signal are caused to beat With locally-generated oscillations in a first detector, and, in general, oscillations of the difference frequency are selected from the first detector, the difference frequency oscillations being fed, after amplification if desired, to a second detector. I

It is well known that in receivers of the superheterodyne type, interference may be caused by the heating with the local oscillations of oscillations at frequencies lying within an image band which is separated from the local oscillation frequency by frequencies of the order of the intermediate frequency; thus if the desired signal has a frequency which is lower than the local oscillation frequency, the image band will lie above the local oscillation'frequency.

Means have already been proposed for reducing interference due to image frequency signals, and it is an object of the present invention to provide new or improved means for this purpose.

According to thepresent invention a superheterodyne Wireless receiver includes tuned circuits coupled by an impedance associated with means for attenuating undesired signals without substantial attenuation of desired signals. The tuned circuits together with the coupling impedance 'may form a .band pass filter and the means for attenuating undesired signals may be designed particularly to reduce the effect of image frequency interference.

In a particular circuit arrangement for a wireless receiver according to the invention, the impedance coupling the tuned circuits consists of a condenser and the means associated therewith for attenuating undesired signals consists of an inductance coil, the values of said condenser and inductance coil being such that the resonant frequency of the combination lies somewhat above the highest frequency to which the coupled circuits are tunable. The said inductance coil may have a second inductance coil connected in series with it, the second coil being shunted by a condenser of such a value that the combination resonates at the low frequency end of the band presented by this circuit is almost purely capacitive. The value of the first mentioned condenser is reduced whenthe second coil and condenser are added in order that the total value of the coupling capacity may be correct for the desired while at the high frequency end the impedance width of pass band. Both inductance coils are coupled to a third inductance coil connected in the aerial lead to the receiver. In order that the invention may be more clearly understood and readily carried into effect, a circuit embodying the invention will now be described by way of example with reference to the accompanying drawing in' which:

Fig. 1 shows the preselector circuits of a superheterodyne wireless receiver, and

Fig. 2 is a constructional example of a coil assembly.

Referring to Fig. 1 of the drawing, the input circuit or preselector of a wireless receiver is shown in the form of a capacitively coupled bandpass filter including two inductance coils l and 2, which are screened from one another by a screen not shown in the drawing. The lower end of coils l and 2 are connected together and earthed through a coupling condenser 3, inductances 9, II and condenser 12. Two tuning condensers 4 and 5 having their rotors ganged together and connected to earth are provided, the stator of the condenser 4 being connected to the free end of the coil I which will be referred to as the aerial coil, while the stator of the condenser 5 is connected to the free end of the coil 2, which will be referred to as the grid coil. A tapping point in the aerial coil circuit is connected to a receiving aerial 6 through a variable condenser l of small capacity, for example of the order of -100 micro-microfarads and an inductance 10.. The junction point of the grid coil 2 and its associated tuning condenser 5 is connected to the control grid of a mixer valve 8. The coupling condenser 3, as will be seen from the drawing, is in series with the condenser l2 between the lower ends of the aerial and grid coils and earth, and the effective capacity of condenser 3 is made of such a value that the desired width of pass band'for the filter is obtained.

The coil 9 is of such a size that with the condensers 3 and 12 the circuit is series resonant at a frequency somewhat higher than the highest frequency to which the filter is tunable. size of the coil II is such that with the shunt condenser l2, this'portion of the circuit is resonant at a frequency at or just below the low frequency end of the band of frequencies covered by the tuned circuits. The coil it in the aerial lead is coupled to both coils 9 and I l and is placed relatively to those coils as shown in Fig. 2 which is an approximately full scale illustration of the coils.

The

In a practical example, the components of the image suppressing unit may be as follows:

Condensers 3 and I 2 microfarad .02 Coil 9 (on former) turns 5 Coil ID (on former) do 30 Coil II (on an approximately former) do 14 The aerial and grid coils I and 2 and the tun ing condensers 4 and 5 are of the size normally used to tune over the medium wave band.

While an image suppressor unit as described has been found very effective in practice, the theoretical analysis of the operation of the circuit is complicated. It is thought, however, that the operation is somewhat as follows.

If reasonably good tuning coils are used the type of capacity coupled band pass filter described is in itself sufficiently selective to provide adequate discrimination against image frequencies at wavelengths above about 300 meters. The image suppression is therefore more necessary at the relatively low wavelengths.

The circuit including coil 9 and condensers 3 and I2 is therefore made to resonate at the high frequency end of the frequency band covered by the filter and voltages set up across the coil 9 are fed through coil i0 to the aerial coil i in antiphase to the incoming signals. The voltages fed back are predominantly of the image frequency because at resonance the impedance of the coupling path is very low compared with the impedance of the tuned circuits while beyond the resonant point, at the higher frequency side, the tuned circuit impedance is low while the impedance of. the coupling circuit is increased. As previously stated, very effective image frequency suppression is obtained by the device and method described, although the strength of the incoming signals is not undesirably reduced by the feed back.

The arrangement, has however, the disadvantage that at the high wavelength end of the frequency band it was found that the previously good selectivity of the circuits had been destroyed to some extent, due probably to the signal being injected directly into the second circuit through the coils in and 9. To obviate this difiiculty the circuit H and I2 tuned to a low wavelength is added in order to inject into coil H] a voltage equal and opposite to the voltage produced by coil 9 at the low wavelength end only. Thus the following conditions obtain with the receiver tuned below about 350 meters. The image suppressing voltage across coil 9 is injected into the tuned circuit I--4 through the coil I0, While above this wavelength the coupling between 9 and I0 is effectively cancelled due to the coil H feeding into coil l0 voltages equal and opposite in phase to the voltages fed to coil ID from coil 9. In other words, image suppression is obtained below about 350 meters and above this wavelength where image suppression is not required the suppression circuit is prevented from having any adverse effect on the working of the filter.

The degree of suppression obtainable is very easily adjusted by relative movement of the coils 9, I0, II on their former l3, as shown in Fig. 2, and they may be fixed in place by means of a suitable varnish. As the additional circuits are of a low impedance, such adjustments are very stable and easily made.

I claim:

1. A superheterodyne wireless receiver provided with a pair of tuned circuits resonant to a desired signal frequency, coupling means therefore, said coupling means comprising an impedance common to both said tuned circuits, and means resonating with said common impedance at the frequency of undesired signals for effecting attenuation of undesired signals without substantial attenuation ofdesired signals.

2. A wireless receiver according to claim 1 wherein said tuned circuits together with said common coupling means form a band pass filter and wherein said means for effecting attenuation of undesired signals is adjusted to suppress or reduce image frequency interference.

3. A wireless receiver according to claim 1, wherein said common coupling means consist of a condenser and the means resonating therewith for attenuating undesired signals consists of an inductance coil, the values of said condenser and inductance coil being such that the resonant frequency of the combination lies above the highest frequency to which the coupled circuits are tunable.

4. A wireless receiver according to claim 1, wherein said common coupling means consist of a condenser and two inductance coils connected in series and a source of signal energy provided with a further inductance coil coupled to said two inductance coils, one of said two inductance coils being shunted by a condenser of such a value that it serves to by-pass frequencies at the upper end of the frequency range to which the coupled circuits are, tunable.

5. A wireless receiver according to claim 1, wherein said common coupling means consist of a condenser and a pair of inductance coils all serially connected together, the values of said condenser and one of said inductance coils being such that the resonant frequency of the combination lies above the highest frequency to which the coupled circuits are tunable, the other inductance coil being shunted by a condenser of such a value that it serves to by-pass frequencies at the upper end of the frequency range to which the coupled circuits are tunable.

6. A superheterodyne wireless receiver comprising a pair of tuned circuits, means for coupling said circuits comprising a condenser and two inductances connected in series with it, said condenser and one of said inductances having values such that their resonant frequency lies above the highest frequency to which the coupled circuits are tunable, said second inductance being shunted by a condenser of such value that it serves to by-pass frequencies at the upper end of the frequency range to which said coupled circuits are tunable, and a source of signal energy provided with an inductance which is coupled to said two serially connected inductances.

7. In a superheterodyne radio receiver, a selector network for selecting currents of a desired frequency and attenuating image frequency currents comprising a pair of coupled tunable circuits adjustable over a predetermined frequency range, each such circuit being constituted by an inductance and a variable condenser, the rotors of said condensers being connected together and to ground, the low potential ends of said inductances being connected together and to ground through a network which comprises a series resonant circuit tuned to a frequency at the high frequency end of the range to which the coupled circuits are tunable and a parallel resonant circuit tuned to a frequency at the low frequency end of said range.

8. A selector network according to claim 7 nected respectively to the signal source and to an intermediate point of the inductance in one of the tunable circuits, and a connection from the high potential end of the inductance of the other tunable circuit to the signal control element 5 v of a vacuum tube.

PERCY ARMSTRONG LE MARCHANT. 

